The present invention relates to an image reading device used as an image read section of a facsimile, scanner, and the like, and more particularly to an arrangement of a capacitor section for temporarily storing the charge as generated by an photoelectric conversion element according to optical density information of each extremely small area on an image of an original.
An image reading device for reading an image on an original in close contact with the original is made up of an array of photoelectric conversion elements that are linearly arrayed, and a drive circuit for driving the array. Charges generated by the photoelectric conversion elements in the array are time sequentially picked up and output to a single output line by means of switches for sequentially selecting the photoelectric conversion elements. To individually drive a number of photoelectric conversion elements of the array, a number of drive IC chips are required. To cope with this, there has been developed an image reading device of the TFT drive type which employs such a parallel processing that switchings of several bits are simultaneously done by a-Si:H thin film transistors (TFT), thereby to reduce the required number of the drive IC chips and hence to reduce the cost to manufacture.
The image reading device of the TFT drive type, as shown in FIG. 16, is made up of a linear array of photoelectric conversion elements whose length is substantially equal to the width of an original, a charge transfer section 52 including thin film transistors T.sub.k,n provided for photoelectric conversion elements in one-to-one correspondence, and a multi-layer wiring section 53.
The linear array 51 consists of K number of blocks each including N number of photoelectric transducing elements 51'. Each photoelectric conversion element 51' may be equivalently expressed by the combination of a photo diode PD.sub.k,n and a stray capacitance CD.sub.k,n. The photoelectric conversion elements 51' are respectively connected with the drains of thin film transistors T.sub.k,n. The source electrodes of the transistors T.sub.k,n are respectively connected, for each block, with common signal lines 54 (N number of lines) and load capacitors CL.sub.n, through multi-layer wires 53 arrayed in a matrix fashion. The gate electrodes of the thin film transistors T.sub.k,n are connected with a gate pulse generator (not shown), for conduction of each block. Photo charges generated by the photoelectric conversion elements 51' are temporarily stored, and then are sequentially transferred, for each block, to and stored in the load capacitors CL.sub.n by means of the thin film transistors T.sub.k,n as charge transfer switches. More specifically, in response to a gate pulse .phi.G1 generated from the gate pulse generator (not shown), the thin film transistors T.sub.1,1 to T.sub.1,n of the first block are turned on. Charges as generated by and stored in those photoelectric conversion elements 51' are transferred to and stored in the load capacitors CL.sub.n. Depending on the charges stored in the load capacitors CL.sub.n, potentials in the common signal lines 54 vary. The voltage values are time sequentially picked up and output to the output line 56 by sequentially turning on the analog switches SW.sub.1 to SW.sub.n. In response to gate pulses .phi.G.sub.1 to .phi.G.sub.n, the thin film transistors T.sub.2,1 to T.sub.2,n, to T.sub.k,1 to T.sub.k,n of the second to K-th blocks are turned on, and the charges from the photoelectric conversion elements are sequentially transferred for each block. In this way, the charges are sequentially read, to obtain an image signal of one line in the main direction of the original. By moving an original feed means (not shown), such as a roller, the above sequence of read operations is repeated, finally to obtain image signals of the whole original image.
A specific operation of the image reading device will be described with reference to an equivalent circuit (FIG. 17) of one bit which includes a single photoelectric conversion element 51'. To set up an initial state, a reset switch RS is closed. In turn, a reverse bias voltage (VB) is applied to the photo diode PD of the photoelectric conversion element 51'. A potential (VL) of the common signal line 54 is reset to 0 V. A light from a light source (not shown) is applied to an original (not shown) put on the photoelectric conversion element array, the light reflected at the original illuminates the photo diode PD. In the photo diode, photo current Ip flows whose amplitude depends on an optical density on the original, and the current causes the diode to generate charges. The charges are stored in a stray capacitor CD of the photoelectric conversion element 51' and an overlap capacitor Cgd between the gate and drain electrodes of the thin film transistor T. In response to a signal .phi.G from the gate pulse generator, the thin film transistor is turned on, and connects the photo diode PD and the common communication line 54. As a result, the charges are transferred to and stored in the load capacitor CL. Since the signal input of the multiplexer has high impedance, all the charges are stored in the capacitors in the circuit. Accordingly, the "charge transfer" means a re-allocation of charges to the capacitors (CD, Cgd) of the diode portion and the capacitors (CL, Cgs) in the common signal line portion. After the potential VL is detected after the transfer completion, the common signal line 54 are reset by the reset signal RS, to transfer the bit signals of the next block.
A structure of the photoelectric conversion element portion of the image reading device is as shown in FIGS. 18 and 19. As shown, a belt-like common electrode 62 made of metal, e.g., chromium (Cr), is formed on an insulating substrate 61. Photoelectric converting layers 63 made of amorphous semiconductor (a-Si, for example), separately every bit, are discretely formed on the common electrode 62. Individual electrodes 64 of transparent conductive films which are made from, for example, indium tin oxide (ITO), are formed on the photoelectric converting layers 63, respectively. An interlayer insulating film 65 is applied over the photoelectric converting layers thus formed. Wires 66 are formed, for each photoelectric conversion element, on the interlayer insulating film 65. The individual electrodes 64 and the wires 66 of the photoelectric conversion elements are interconnected through contact holes 67 each formed in the interlayer insulating film 65 above the end portion of the individual electrode 64.
With such a structure of the image reading device, charges generated in each photoelectric conversion element 51' are temporarily stored in the stray capacitance CD of the photoelectric conversion element 51' and the overlap capacitor Cgd between the drain and gate electrodes of the thin film transistor T. To improve the switching characteristic of the thin film transistor, it is necessary to reduce the overlap capacitor Cgd. The stray capacitance CD of the photoelectric conversion element 51' is determined by an area of a stray capacitance portion 68 (slanted area in FIG. 18). This portion is a shaded portion of each area of the image reading device where the photoelectric converting layer 63 is sandwiched between the common electrode 62 and the individual electrode 64. To improve the resolution with less interference between adjacent bits, the area of this portion must be reduced. For these reasons, it is difficult to secure the stray capacitance CD and the overlap capacitance Cgd large enough to store the generated charges.
Since the structure requires dielectric material for the semiconductor (a - Si), dielectric constant of the semiconductor varies depending on voltage application and exposed light amount. Therefore, the capacitance of the stray capacitor CD is unstable.